The present invention relates to a rotational speed signal sensor for an electric motor having a rotating permanent magnet as a rotor.
A typical prior art rotational speed signal sensor of the type described consists of a gear which is made of magnetic materials and mounted on the shaft of a DC motor, an exciting magnet and a magnetic sensor head mounted on the frame of the motor. Upon rotation of the motor, the magnetic gear causes the magnetic path between the sensor head and the magnet to vary so that the output signal of a frequency representative of the rotational speed of the DC motor may be derived from the output terminals of the sensor head. However the rotational speed signal sensor of the type described above is disadvantageous in that it is complex in construction and needs a large installation space because it is installed at the exterior of the motor and that the productivity is low because of a large number of components.